From a materials perspective, the manufacture of olefins by hydrocarbon steam pyrolysis has not changed very much since originally commercialized, except to progressively operate at higher operating temperatures with overall greater cracking severity. Process containment or furnace coils have evolved in alloy composition and properties over the last 60+ years to sustain the higher temperatures and lower feedstock residence times. This has resulted in an increase in unwanted or negative catalytic reactions at the coil surfaces and other carbon-based fouling mechanisms; for example, carbon or coke build-up by surface-catalyzed “filamentous” coke-make. Overall, these fouling mechanisms reduce furnace and plant efficiencies, and significantly increase furnace maintenance costs.
Efforts aimed towards mitigating the problem have made some progress over the last quarter century. These efforts include better alloys and coil surfaces, feedstock additives and inhibitors, and coil coatings. For example, in the 1980s and 1990s, several promising coating technologies were developed and commercialized aimed at rendering the internal surfaces of furnace coils chemically inert to the pyrolysis or cracking process (i.e., shutting-down catalytic or filamentous coke-make). Overall, these coatings were able to provide some improvements in furnace run-lengths from a typical baseline of ˜20-40 days. The use of inert coatings increased run lengths by a factor of 2-3 times. The run lengths, however, rarely exceeded ˜100 days on-line. The success of some of these coatings prompted some steel producers to develop and commercialize novel alloys away from industry-standard chromia-forming austenitic stainless steels whose surfaces exhibit relatively low temperature stability under cracking conditions (<1050° C. (1922° F.)). The newly developed steels were engineered with higher temperature-stable surfaces through the use of alumina-formers.
Hydrocarbon processing in the manufacture of petrochemicals is carried out in processing equipment that includes tubing, piping, fittings and vessels of broad geometries and alloy compositions. These components are generally made of ferrous-based alloys designed to provide adequate chemical, mechanical and physical properties for process containment, and resistance to a range of materials degradation processes. In commercial applications operating above 500° C., austenitic stainless steels are often used ranging from 300 series alloys through to 35Cr-45Ni—Fe alloys, with the level of nickel and chromium in the alloy generally increasing with operating temperature. Above 800° C., a sub-group of these austenitic steels are used and are collectively known as high-temperature alloys (HTAs) or heat-resistant alloys. These HTA steels range from 25Cr-20Ni—Fe (HK40) through to 35Cr-45Ni—Fe (or higher), plus alloying additives in cast form, and similar compositions in wrought form. In general, stainless steel surfaces are prone to the formation of filamentous (catalytic) carbon or coke and the accumulation of amorphous (or gas-phase) coke, with their relative contribution to the total coke-make being defined by the petrochemical manufacturing process, feedstock, and the operating conditions. Filamentous coke formation is well documented and has been shown to be catalyzed by transition metal surface species, their oxides, and compounds thereof, with iron and nickel-based species being the major catalysts present in stainless steels.
The broad commercial use of stainless steel alloys, especially HTAs is partially due to their ability of generating and re-generating a protective rhombohedral chromia (Cr2O3) scale for protection. These steels are collectively known as “chromia-formers” with the scale believed to provide both corrosion protection and resistance to filamentous (catalytic) coke formation. It is generally accepted that a bulk alloy level of 13-17 wt % Cr is required to generate and sustain a contiguous and protective chromia scale. The overall protection provided by the chromia is good to excellent within its operating limitations. One critical limitation pertinent to hydrocarbon processing is that under highly carburizing conditions (as for example with a carbon activity ac≧1 during steam pyrolysis of aliphatic hydrocarbon feedstock) and temperatures greater than approximately 1050° C. (or lower depending on actual conditions), the chromia is converted to chromium carbides, leading to volume expansion, embrittlement, and subsequent loss of protection. Additionally, under highly oxidizing conditions (as for example, during furnace start-up and decoking), above a critical temperature, the chromia is converted to CrO3 and volatilized. Therefore, there is great commercial value in a base alloy with the mechanical and physical properties of the HTAs currently used, but with a protective coating and surface that overcomes the limitations of the chromia scale and provides greater protective benefits for reducing carbon-based fouling and corrosion.
In the manufacture of major petrochemicals, the generation of a chromia scale on process components such as furnace coils is often critical in achieving and perhaps exceeding furnace design capacity. As an example, in steam pyrolysis of ethane to produce ethylene, the operating sequence is typically 20-90 days online of production, followed by 1-4 days offline for decoking. This industry “optimum” capitalizes on the protection provided by the chromia scale, while operating, as best as is feasible, within the chemical and mechanical limitations that the chromia scale imposes on the process.
Efforts to reduce filamentous (catalytic) coking have involved the use of coatings, pre-oxidation of components, chemical additives, or a combination thereof, all aimed at rendering the surface catalytically-inert to filamentous coke-make. Examples of coated products are based on the teachings of U.S. Pat. No. 5,873,951 and Canadian patent 2,227,396 aimed at generating an alumina layer in contact with the process stream. Canadian patent 2,227,396 also teaches the use of a coating aimed at generating a chromia layer at the outermost surface. U.S. Pat. No. 4,297,150 teaches the use of CVD processes to deposit coatings aimed at providing a silica layer in contact with the process stream. The use of chemical additives in some petrochemical industries is broad. As an example, most commercial operations manufacturing olefins by steam pyrolysis add a sulfur-based compound (such as DMS or DMDS) to the feedstock at levels of a few ppm to several hundred ppm to poison catalytic surface sites. Alternatively, other efforts have tried to passivate the surface through the addition of various proprietary chemical additives to the feedstock (see U.S. Pat. Nos. 4,613,372, 4,804,487, 4,863,892, 5,015,358, 5,565,087, 5,616,236, and 5,446,229). Generally, the level of commercial success achieved through the use of coated products, pre-oxidation, or chemical additives to reduce filamentous (catalytic) coking in light feedstock olefins furnaces has generally been limited to a 2-3 fold improvement in run-length at best, over industry surveyed run-lengths that were presented at the AIChE Ethylene Producers' Conference in 1995. Most recently, NOVA Chemicals (see U.S. Pat. Nos. 5,630,887, 6,436,202, 6,824,883, and 6,899,966) has achieved run-lengths in excess of 400 days (better than a 10-fold improvement in runlength) with a gas treatment technology based on generating a [Cr—Mn]-spinel surface on the steel components, and SK (see U.S. Pat. No. 6,514,563 and U.S. Pat. No. 6,852,361) has achieved a 3-4 fold improvement with an in-situ coating application technology.
The selection and use of protective surface oxides on stainless steels by the above teachings is illustrated in Table 1 hereinbelow (see Metallurgical and Materials Transactions A Vol. 11 Number 5, May 1980 Tritium permeation through clean incoloy 800 and sanicro 31 alloys and through steam oxidized incoloy 800 Author(s): J. T. Bell; J. D. Redman; H. P. Bittner Pages: 775-782; and Analysis of oxide coatings on steam-oxidized incoloy 800 Author(s): H. F. Bittner; J. T. Bell; J. D. Redman; W. H. Christie; R. E. Eby Pages: 783-790) with efforts aimed at generating surface species more thermodynamically stable than chromia. Commercially-available furnace products used in the manufacture of petrochemicals have focused mainly on providing a chromia, silica, alumina or a [Cr—Mn]-spinel scale in contact with the hydrocarbon process stream.
TABLE 1Relative Oxide Stability of Austenitic StainlessSteel Components from Free Energies ofFormation DataOxide−ΔGo × 10−4 (cal/mole O2) at 900KNiO 7.45Fe2O3 9.35Fe3O4 9.85FeO 9.88Mn2O311.58Mn3O412.78FeCr2O413.34Cr2O314.35MnCr2O4N/AMnO15.26SiO217.10Ti2O320.19Al2O322.15
In summary, the prior art related to materials solutions (coatings, modified base alloy formulations, or pre-oxidation) to the coking, catalytic activity and corrosion problem in petrochemical furnaces teaches that stainless steel alloy technology is based on generating a chromia protective scale, and that recent teachings suggest that similar austenitic HTAs can also be used to generate an alumina, silica or Cr—Mn spinel. Secondly, with the exception of the NOVA Chemicals [Cr—Mn]-spinel technology, the prior art teaches that efforts aimed at generating [Cr—Mn]-spinel based surfaces are of little commercial value due to their low thermo-mechanical stabilities and reduced protection to the base alloy after any damage/delamination. Thirdly, it teaches that commercial coated products are based on the generation of a protective alumina or silica scale with other properties that may be superior to the same scale generated on uncoated alloys. Overall, all of the above teachings are aimed at enhancing the inertness of the surface to the cracking process.
The prior art relating to coatings aimed at enhancing the catalytic gasification properties of the surface teaches that carbon gasification during cracking is possible through the use of coatings but little commercial success has been realized to-date primarily due to such products' inability to address survivability requirements under the extreme conditions present in olefins manufacture.
The disclosure hereinbelow capitalizes on the potential negative impact on the overall cracking process, despite the relatively low surface area exposure to the overall process stream, and provides coatings and surfaces that can eliminate the unwanted (negative) catalytic properties as one benefit, and simultaneously provide positive or beneficial catalytic activity as a major new materials and process benefit to the industry. Such coatings and surfaces can provide significant commercial value ranging from improvements in plant efficiencies and profitability, to reducing energy requirements, steam dilution requirements and overall greenhouse gas emissions.
The disclosure hereinbelow involves the application of functionally-graded coatings that sustain surfaces with positive catalytic activity, and a range of catalyst formulations and surface loading integrated into commercially-viable coating systems using current industry furnace alloys. Two families of surfaces have been developed, providing a significant range of catalytic functionality impacting the process, as well as a coating system aimed at ensuring commercial viability. The coatings are best described as composites, consisting of metallic, intermetallic and ceramic constituents, and exclude expensive constituents such as precious metals. It is recognized that olefins furnaces represent some of the most extreme high temperature and corrosive conditions of any industrial manufacturing and represent serious challenges to commercial-scale viability. Overall, the disclosure herein aims to provide additional chemical, physical and thermo-mechanical properties in its coatings to achieve commercial viability.